The presence of each member, when thus separated is then confirmed by one of its special tests.

222. (s) A solution which may contain Pb, Ag, Hg' is examined by first precipitating by HCl added in excess, then filtering and adding more HCl to the clear filtrate to ascertain that no further precipitate is produced. The precipitate is then examined by Table I. (433.)

The foregoing reactions only include the metals of common occurrence. For information concerning the detection and separation of the rarer metals, which will only be required by a somewhat advanced student, the appendix or larger analytical treatises must be consulted. The metals gold and platinum, however, are briefly noticed here on account of their frequent employment for chemical and other purposes. These metals are dissolved only by a mixture of HCl and HNO3: their most important reactions are given below.

GOLD (Au).-Use AuCl, solution.

Gold solutions are usually bright yellow in colour.

223. SnCl, containing a little SnCl4: purple or dark-brown precipitate ("Purple of Cassius "), best seen by performing the experiment in a white porcelain dish.

224. FeSO4 produces either at once, or on heating the solution, a very finely-divided precipitate of Au: the liquid usually appears bluish by transmitted light; always copper-red by reflected light :

2AuCl2+6FeSO4=2Au+2Fe2(SO4)3 + FeClg.

224a. H2SO, produces on boiling a similar precipitate of Au: by boiling the liquid for some time in a porcelain dish, the Au settles in small black masses and the solution loses its colour.

PLATINUM (Pt).-Use PtCl solution.

225. AmCl added to a strong solution produces on standing for some time, or more quickly on being stirred or evaporated, a yellow precipitate (Am,PtCl): rather soluble in hot water.

226. SnCl2 gives a reddish-brown colour in the solution acidified with HCl, owing to the formation of PtCl2:

PtCl + SnCl2 = PtCl2+SnCl4.

The method of separating and detecting Au and Pt is fully stated in paragraphs 472–474.

*Note.-Before commencing the reactions for acids the student may with advantage analyse several substances which are liable to contain any one or more of the members of the metallic groups. If only one metal has to be looked for (see column 1, paragraph 539), the directions given in paragraphs 330-369 may be followed, omitting those portions which relate to the detection of acid-radicles. If two or more metals may be present (see columns 2, 3, paragraph 539), full directions for ascertaining to which analytical group or groups they belong will be found in the General Table (419); and the group precipitate or precipitates thus obtained may be then examined by the Group Tables (433-439), with which the student will probably already be familiar.

REACTIONS FOR ACID-RADICLES.

In the course of analysis acid-radicles are usually detected by special tests: they cannot advantageously be precipitated in groups and the members of each group then separated and identified, as is done in the case of metals. Accordingly the arrangement here adopted consists in simply placing together in a group such acid-radicles as in some respects resemble one another in their reactions, and at the end of each group stating upon what differences the detection of its members when occurring together depends.

The reactions for acid-radicles are worked through in a manner similar to that employed for the reactions of the metals,—a compound containing the radicle (usually a salt) being employed. At the end of each group the student may with advantage detect one or more of its members as was done with the metals.

GROUP I.-SULPHATE GROUP.

The sulphates are the only commonly occurring salts which give with BaCl, a precipitate insoluble in boiling HCl. Hydrofluosilicates resemble sulphates in this respect, but differ in so many other reactions that they are considered hereafter (299-302) in connection with fluorides and silicates, to which they are more closely related.

SULPHATES ("SO4).-Use Na2SO4.10H2O.

227. BaCl2 [or Ba(NO3)2]: white precipitate (BaSO4), insoluble when boiled with HCl or HNO3.

Note.-If BaCl, or Ba(NO3)2 be added to a solution containing much strong HCl or HNO3, a white precipitate may be produced consisting of BaCl, or Ba(NO3)2, which, however, is readily distinguished from the precipitate of BaSO4 by being dissolved when boiled with water. Sr(NO3)2 and Pb(NO3)2 give also white precipitates (SrSO, and PbSO4).

228. Blowpipe Test.-Solid Na,SO4 or any solid sulphate, if mixed with solid Na,CO,* and fused on charcoal in the inner blowpipe flame produced from a spirit-flame, yields Na,S:

Na2SO4+4C = Na2S+4CO.

The Na2S is readily detected by detaching the solid mass, after it has cooled, from the charcoal with the point of a knife; placing a portion of it upon a bright silver coin, then moistening it with a drop of water and crushing it by pressure with a knife-blade. After the moistened mass has remained upon the coin for a short time rinse it off, a black stain will then be seen upon the silver where the substance had rested.

Another portion of the cool mass may be moistened with a drop of HCl, when the fetid smell of HS will be observed, and a piece of paper moistened with PbA, solution and held above it, will be blackened (230).

Note. This test for a sulphate is reliable only when it is known that sulphur in no other form is present in the substance to be tested; it is also necessary that no sulphur should be introduced in the Na, CO, or by the flame. Now coal-gas sometimes contains sufficient sulphur to give the above reaction with pure Na,CO,, hence it is always advisable to employ a spirit- rather than a gas-flame.

Hydrogen sulphate, or sulphuric acid (H2SO4), may be detected by producing black charred stains on paper which has been dipped into it and dried by heat: it also has a strong acid reaction to litmus, and evolves hydrogen with Zn, properties shown by some metallic and acid sulphates.

Addition of Na2B4O7 is useful in preventing the fused mixture from sinking into the charcoal.

GROUP II. THE CARBONATE GROUP,

The five following classes of salts give off characteristic gases when acted upon by HCI.

*** The systematic Tables of Differences, such as are given for each group of the metals, are not drawn out for the acidradicle groups; the student should have no difficulty in constructing such tables for himself, either mentally or on paper, by looking through the reactions.

CARBONATES ("CO).-Use Na,CO,, or white marble (CaCO3).

3

229. HCl or HÃ or almost any other acid, if poured upon some Na,CO, in a test-tube, causes carbon-dioxide gas (CO2) to come off with effervescence. This gas is recognised by its property of turning lime- or baryta-water milky by the production in them of insoluble CaCO3 or BaCO3.

The test may be tried in several ways :—

1. The acid is poured upon the carbonate in a test-tube. A glass rod which has been dipped into some perfectly clear lime-water* and has a small drop adhering to its end is then introduced into the test tube, carefully avoiding touching the liquid or the sides of the tube; the drop will quickly become milky.

2. The acid is poured upon the Na,CO, in a small beaker, and this is immediately covered with a watch-glass placed with its convex face downwards, having a drop of clear limewater adhering to the middle of the face: the drop becomes milky.

3. Since CO, gas is much heavier than air, on adding acid the gas evolved from the carbonate will remain in the testtube if the tube is held erect and its mouth loosely closed with the thumb: by gradually sloping the tube the heavy gas is poured into another test-tube containing lime-water without allowing any liquid to flow out, on closing this tube with the thumb and well shaking it the lime-water will

Baryta-water gives a much more decided result than lime-water,

become milky. Or if the inside of the tube has been rinsed with lime-water the sides become white.

FIG. 37. FIG. 38.

4. The CO2 if it is given off in considerable quantity, may also be made to pass into some lime-water through a doubly bent tube fitted air-tight by a perforated cork or india-rubber stopper into the tube in which the CO2 is generated, as shown in the fig. 37; or the apparatus shown in fig. 38 may be employed.

Of the above methods 1 and 3 are the most simple; by method 4 the CO2 is not found unless given off in somewhat large quantity.

Note. The following precautions must be attended to. Since the milkiness at first caused by CO2 disappears gradually when more CO2 is absorbed, if the lime-water does not at once become milky, it must be constantly watched to avoid the risk of its becoming milky and again clear before being seen.

Also if the addition of the first few drops of acid does not cause an effervescence of CO, more acid should be added, as many carbonates retain the first portions of CO2 by forming acid carbonates:

Na2CO3 + CO2 + H2O=2NaHCO ̧ .

A solid substance which is being tested by HCl for CO2 should be first moistened so as to drive out the air-bubbles entangled in it, which, in coming off, might make one suspect effervescence of CO2.

Hydrogen carbonate, or carbonic acid (H,CO), can only exist in dilute aqueous solution; addition of lime-water to its solution causes milkiness; but, since other substances in solution behave in a similar way, it is best found by boiling the liquid and passing the steam with the CO, which accompanies it into lime-water; the lime-water becomes milky.

SULPHIDES ("S).-Use FeS for solid, and Am,S for liquid. 230. HCl (or H2SO4) poured upon a small fragment of FeS causes HS gas to come off with effervescence: this gas is detected by its fetid smell, resembling that of rotten eggs,